The present invention relates to a process for the production of hydrogen/deuterium-containing gas from hydrocarbons or coal and steam or oxygen or oxygen-bearing gas. Such a gas serves for the recovery of deuterium, the latter being separated from the hydrogen/deuterium-containing gas in socalled deuterium enrichment plants. The hydrogen-containing gas depleted in deuterium can be further used, for example for the production of ammonia.
In these processes for the production of hydrogen/deuterium-containing gas it is necessary to attain the maximum possible deuterium concentration in the gas because the volume of process equipment, and, consequently, the costs for the recovery of the deuterium depend on said concentration.
It is known that the deuterium content of both hydrocarbons and water is almost independent from their origin. This content is commonly known as the natural deuterium content. It is in the order of 145 ppm of deuterium referred to hydrogen. The range of variation of that natural concentration does generally not exceed 5 ppm. Theoretically, the deuterium concentration in the hydrogen produced should correspond to the deuterium concentration in the hydrocarbons fed to the cracking process and to the deuterium concentration in the steam fed to the cracking process.
It is known to produce hydrogen/deuterium-containing gas by the autothermic or allothermic cracking of hydrocarbons with addition of steam and/or oxygen. Known processes also include those where this gas is produced by the gasification of coal. In any case, the gas contains carbon monoxide and carbon dioxide apart from hydrogen. In the CO-conversion step that follows the cracking process, the carbon monoxide is converted to carbon dioxide with the aid of steam while hydrogen is produced at the same time. Generally, the gas obtained in this way does not have the composition that is required for final processing.
Therefore, it is subjected to further process steps for the removal of carbon dioxide and for eliminating components harmful for the downstream process. Depending on the process pressure applied, the gas obtained can be sent to a deuterium enrichment plant either directly or after compression. This deuterium enrichment plant may be a hydrogen distillation facility or may operate along an isotope exchange process e.g. operated according to the hydrogen/ammonia or hydrogen/water system. After having been depleted of deuterium in the deuterium enrichment plant, the hydrogen is further processed, for example in an ammonia synthesis plant.
The production of deuterium or heavy water of the deuterium enrichment plant is limited by the quantity of hydrogen, the concentration of deuterium in the hydrogen fed to process, and the deuterium yield of the process.
Measurements performed on the hydrogen/deuterium-containing gas have proved that the deuterium content is substantially lower than the natural content. Readings were partly below 100 ppm. Extensive studies have evidenced that this depletion (decrease in the original deuterium content of the fed components) in deuterium of the hydrogen is attributable to an isotope exchange between hydrogen and the water vapour present in the gas. The water vapour contained in the gas mixture produced contains, indeed, substantially more deuterium than is found as natural content in the feedwater. This phenomenon was evidenced numerically by a deuterium balance. The studies also showed that this depletion occurs primarily in the carbon monoxide shift converstion. This process step involves a high portion of water vapour, and the gas temperature has dropped to a point where the isotope distribution in the hydrogen and water vapour has markedly shifted to the water vapour side. This enriched water vapour is condensed in the gas coolers downstream of the conversion section. The condensate is withdrawn and rejected because it generally contains dissolved carbon dioxide and traces of catalyst dust. The economic aspects of this depletion are evident. They involve not only a substantial loss of production but also require more elaborate separation methods because the concentration procedure in the deuterium enrichment plant starts from a feed gas that has a lower-than-natural deuterium content with a consequent rise in production costs.
It has sometimes been suggested, therefore, to reverse this depletion by returning the enriched condensate to the process, that means processing the condensate and returning it to the vaporizers.
As mentioned before, this method would require the return of all condensate to the cracking process, that means the water would first have to be sent through a water treatment plant. This treatment plant would be a very complicated and voluminous facility because of the CO.sub.2 content and the presence of other components, such as sulphur, catalyst dust, etc. Feedsteam for the gas production unit is generally waste steam, i.e. it was produced as superheated steam of about 100 atm.g. in the wasteheat sections of the total plant and reduced in expansion turbines to the process pressure of the gas production unit. The feedwater and steam, respectively, for such waste heat sections and machines must be of high quality.